As shown in FIG. 5, in a conventional method for desalinating salts-containing water (hereinafter, description is made by taking the case of seawater unless otherwise indicated) by utilizing the reverse osmosis membrane method, there has generally been employed a system in which salts-containing water (hereinafter referred to as raw water) to be supplied to a reverse osmosis membrane apparatus: 6 is previously pretreated. Here, pretreatment refers to 1 "filtration" treatment for removing suspended solid in raw seawater, which become a factor of increasing pressure loss of the reverse osmosis membrane apparatus: 6 (in general, said suspended solid is coagulated and made coarse by adding a chemical, for example, a coagulant: CL2 such as FeCl.sub.3, etc. and then applied to a sand filter: 3), 2 "prechlorination" treatment (This is to kill target microorganisms. In general, before the "filtration" treatment, a chemical, for example, Cl.sub.2, NaClO, etc.: CL1 is added. Further, when a reverse osmosis membrane which is easily damaged by a chemical for prechlorination is used, it is necessary to add a chemical for reducing said chemical, for example, a reducing agent such as NaHSO.sub.3, etc.: CL3.), etc. When such a pretreatment unit is integrated into a whole system, increase in initial cost (including the cost of a pretreatment apparatus itself as a matter of course and also the costs of a flow rate control valve for adjusting the treatment amounts of the pretreatment apparatus and the reverse osmosis membrane apparatus and their electricity and instrumentation) is caused as a matter of course, and not only increase in running cost (the cost of chemicals to be added and also the power cost of a pump: 2 for overcoming the resistance of the pretreatment apparatus itself and the resistance of a flow rate control valve (not shown in the figures) provided on a line: C) but also increase in manufacturing cost of produced water are caused.
Further, in the above pretreatment, the suspended solid in the raw water and floc of the coagulant: CL2 added for coagulating said suspended solid and making them coarse captured in a filter medium should be discharged periodically to the outside of the system. In the aspect of protecting environment, these should be treated as wastes.
When the pressure loss of the reverse osmosis membrane apparatus: 6 exceeds a predetermined value, production of fresh water is temporarily stopped, and said reverse osmosis membrane apparatus is cleaned up. Such cleaning is carried out by using a cleaning unit provided separately from the system for producing fresh water shown in FIG. 5 and circulating a cleaning liquid containing ammonium citrate or an organic detergent from said cleaning unit through a course of a line: D2.fwdarw.said reverse osmosis membrane apparatus (a raw water side).fwdarw.a line: F1 by using an exclusive pump. In this cleaning system, separately from the system for producing fresh water, not only the cleaning unit should be provided, but also an exclusive cleaning liquid should be prepared. This point is also a factor of increasing the cost of the whole system. The effect bought about by such cleaning is exerted only on substances which are dissolved in the cleaning liquid, such as a metal hydroxide, etc.
Further, as a novel cleaning method, there have been also proposed methods of making fresh water flow backward from a treated water side to a raw water side by utilizing a positive osmotic phenomenon (Japanese Patent Publication No. 40232/1979, Japanese Provisional Patent Publication No. 59312/1988 and Japanese Provisional Patent Publication No. 119306/1989). However, in these proposals, backward flowing of fresh water is utilized merely as physical peeling force of fine particles adhered to a membrane surface, and the cleaning effects thereof are not sufficient.